Passive intermodulation (PIM), commonly referred to as “rusty bolt effect,” is the generation of unwanted signals by the non-linear mixing of two or more frequencies in a passive device, such as a connector or cable, that normally behaves linearly but when subject to high radio frequency (RF) power tones behaves nonlinearly. Non-linear behavior can result, for example, from a loose or corroded connector or from the close proximity of the device to oxidized metal. Non-linear behavior affecting a system emitting RF signals can also result from the re-radiation of emitted signals, for example from antennas that are outside of, and unrelated to, the system. PIM is created when two signals from different systems or the same system combine and are then reflected at a PIM source, and can be reduced or eliminated by replacing the PIM source. For example, PIM is commonly resolved by replacing faulty cables or connectors, or by removing re-radiating sources.
PIM has surfaced as a problem for cellular telephone technologies such as Global System for Mobile Communications (GSM), Advanced Wireless Service (AWS) and Personal Communication Service (PCS) systems. Cable assemblies connecting a base station to an antenna on a tower typically have multiple connectors that can act as sources of PIM that interfere with cellular system operation. Further, a cellular system may be located near re-radiating sources that can act as sources of PIM that interfere with operation. If the generated PIM harmonic frequency components fall within the receive band of a base station, they can effectively block a channel and make the base station receiver think that a carrier signal is present when one is not. Generally the harmonics of concern are third, fifth, and seventh order, where the third order is of greatest signal strength, and therefore, of primary concern.
PIM has also surfaced as a problem for lower power systems such as Distributed Antenna Systems (DAS) which comprise networks of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS is often deployed to an environment with many potential sources of PIM through re-radiation of wireless signals.
Test systems are available to measure PIM, for example, by creating signals at two different frequencies, amplifying them, and providing them to a device under test (DUT). A return signal carrying the PIM is filtered to select a desired test frequency harmonic where PIM can be detected and PIM magnitude and a measurement of the distance to the PIM source is provided to a technician, enabling the technician to locate and replace the PIM source. Such a system is described, for example, in U.S. Pat. No. 8,294,469 entitled “PASSIVE INTERMODULATION (PIM) DISTANCE TO FAULT ANALYZER WITH SELECTABLE HARMONIC” issued to Bradley and incorporated herein by reference. PIM is measured in DAS with test systems used to measure PIM in cellular systems, but at lower powers to more closely match the operating powers of the DAS.